Mini-Turbos are techniques used in every Mario Kart title after Super Mario Kart. It can only be done while drifting. The method varies slightly for each game, but the result in all cases is a short, but useful boost of speed.
Mario Kart 64
In Mario Kart 64, a Mini-Turbo is performed by executing a drift and turning the kart in the opposite direction it is drifting, then back again. (If drifting to the left, tilt the Control Stick right, then left again, while holding the drift.) If done correctly, the trail of smoke behind the kart will turn yellow. Repeat the previous step until the smoke turn red, then release the drift to launch the boost.
Drifting for too long will cause the kart to skid and spin out, slowing the player down.
Unlike in later games, the Mini-Turbo in this game does not instantly boost the kart's speed. Instead, it increases the engine's power to achieve greater acceleration. This acceleration will last as long as the player didn't slow down by any means ( e.g. going off-road, bumping other racers, hitting an item/hazard, braking, etc.)
Mario Kart: Super Circuit
In Mario Kart: Super Circuit, Mini-Turbos were charged automatically by drifting in a 90-degree angle for more than a second and released by going straight afterwards.
Also like in Mario Kart 64, the Mini-Turbo in this game increases the acceleration until the player slows down by any means.
Mario Kart: Double Dash!!
In Mario Kart: Double Dash!!, the method is roughly the same as it was in Mario Kart 64. In this case, there are sparks instead of smoke, the color order is different (rather than gray-yellow-red, it's yellow-red-blue), and it can be done much faster due to the lack of hopping. When performing a Mini-Turbo in Double Dash!! in midair, the boost can still be obtained upon landing if done correctly. This is not possible in Mario Kart Wii, as Mini-Turbos in that game are entirely based on the length of a successful drift.
Mario Kart DS
In Mario Kart DS, the method is the same as it was in Double Dash!!. However, due to the introduction of kart stats, each kart has a different degree of effectiveness.
Mario Kart Wii and Mario Kart 7
In Mario Kart Wii and Mario Kart 7 the method is made much simpler: drift until blue sparks appear, then stop drifting. In Mario Kart Wii and Mario Kart 7, only karts can perform the longer Super-Mini-Turbo by drifting until the sparks turn orange/red. A Burnout Boost (also for Wii only) can be performed if the racer has stopped by holding both the accelerate and brake buttons until blue sparks appear. Mini-Turbos cannot be performed if drifting is set to "Automatic" (Wii and 7)
A Mini-Turbo boost can also be obtained by performing a Trick Jump over ramps and track edges.
In Mario Kart 8, Bikes now can achieve a Super-Mini-Turbo, and for Karts, it can also be charged by skidding as result of tight steering for an extended time. However, this skidding can only occur in 150cc and 200cc class.
With the exceptions of Super Circuit and Wii onwards, Mini-Turbos allowed a now-outlawed technique known as snaking.
Mario Kart 8
The drifting and intensity of the mini-turbo is now dependent on angle, not allowing for snaking. If you drift in a tight radius, you will go through the stages faster than if you do a wide drift with a large radius.